Determination of filter parameters in an inverter

ABSTRACT

A switching arrangement of an inverter with a filter circuit and a grid relay. For the filter circuit use is made of an equivalent circuit consisting of effective filter inductance, from filter inductance and topology of the filter circuit and effective filter capacitance, from the filter capacitance and topology of the filter circuit. The effective filter inductance and the effective filter capacitance are system parameters. To determine system parameters, a voltage pulse is applied between a first conductor output and a second conductor output when the grid relay is open; the first conductor output and the second conductor output are connected via the switching arrangement to form a closed oscillating circuit a current value of the effective filter inductance and the effective filter capacitance is determined from a current curve and/or voltage curve in the resonant circuit as system parameters for controlling the switching arrangement.

The present invention relates to a method for controlling a switchingarrangement of an inverter with a control, and to an inverter with acorresponding system control with a control.

The switching arrangement of inverters usually includes a bridgecircuit, for example in the form of an H bridge or similar topologies,which generates an alternating current at the alternating current sidein the corresponding alternating current-carrying lines by the clockedswitching of the semiconductor switches contained therein. The switchingarrangement can be controlled, for example, in the sense of a pulsewidth modulation in order to achieve a good approximation of a desiredalternating current form (usually a sinusoidal form).

Since only rectangular pulses can be generated by the switchingarrangement, the switching arrangement is followed by a filter stage ineach phase, which smoothes the alternating current signal via anarrangement of capacitors and chokes (inductances) to approximate thedesired sinusoidal form.

The theoretical foundations for such filter stages are well known and donot constitute an obstacle to optimizing the nominal values of thesecomponents. However, in practice there are frequently problems. Inparticular, the actual specific values of the components often differfrom the known nominal value and the specific values can change overtime due to aging or changing environmental conditions.

For example, a filter stage in a single-phase or two-phase inverter canconsist of at least one inductance and at least one capacitor.Three-phase inverters usually have at least three capacitances and threeinductances. The specific values of these components have a greatinfluence on the control behavior and the oscillation tendency of aninverter, because these components are system parameters of the controland thus influence the control behavior.

The capacitor also has an influence on the control of the reactive powerof the inverter. The specific values of these components are stored inthe control of the inverter and influence the controller parameters ofthe implemented controller. If the control is now based on incorrect orinaccurate specific values, unwanted oscillations, stability problemsand deviations in reactive power can result. For example, if thespecific values are incorrect, harmonics and distortions in thealternating current cannot be compensated for.

In order to be able to comply with the narrow limit values of thereactive power output of the various national guidelines and to be ableto control the inverter optimally, precise knowledge of the componentvalues and thus of the system parameters of the filter stage isrequired.

The component values are usually measured when the inverter ismanufactured and stored as parameters in the control. These work stepsare relatively complex and no aging effects or other operatinginfluences that change the component values (e.g. temperatureinfluences) can be mapped. In order to achieve a specified reactivepower output, an additional current measurement can be carried out atthe grid-side output of the inverter, but this involves considerableeffort.

With currently known methods from the prior art, it is possible todetermine exact component values of capacitances and inductances. Thesemethods are usually only suitable for a specific topology. In addition,these methods also reach their limits as the number of componentsincreases.

EP 3232217 A1 discloses a method for determining current values offilter capacitances, wherein when a filter capacitance is charged anoscillating circuit is produced via the semiconductor switches whichincludes the filter capacitance(s). The oscillation in the oscillatingcircuit driven by the initially charged filter capacitance is evaluatedto determine a current value of this filter capacitance. The values ofthe filter inductances required for the determination are assumed to beunchangeable and known. Determining the filter capacitances is used tomonitor the condition of the filter capacitances. In particular, acomparison with stored nominal values is used to check whether or notthe filter capacitances are still ready for use. Control of theswitching arrangement cannot be improved in this way.

EP 3069158 B1 discloses a method for determining capacitance values ofcapacitances in a power supply system, in which a three-phase inverteris operated with open grid relays to set up an island grid. At least twooutputs of an inverter bridge are supplied with an in-phase AC voltage.Current capacitance values of the filter capacitances and/orintermediate circuit capacitances are determined from the currentsflowing at the outputs of the inverter bridge and at least one voltagepresent at the intermediate circuit capacitance and/or a filtercapacitance. Here, too, the determination of the capacitance valuesserves to check a operability of the filter capacitances. Control of theswitching arrangement cannot be improved in this way.

One of the objects of the present invention is to provide a method bywhich the control of the switching arrangement of an inverter with afilter circuit can be improved.

According to a first aspect, these and other objects are achieved by amethod according to claim 1 and by an inverter with a correspondingcontrol according to claim 10.

This method allows the system parameters of the filter circuit to bedetermined for a large number of different inverter and filter circuittopologies. For this purpose, an overall effect of one or morecombinations of interconnected components is determined without havingto know a specific combination or individual component values. Thecurrent curve can be measured at one of the inductances, for example.The determination of a voltage curve can preferably be measured betweenthe two conductors involved in the oscillating circuit or from one ofthe conductors with respect to another reference potential, for examplea star center point or an intermediate circuit center point. Knowledgeof the effective filter inductance and filter capacitance is sufficientfor the control. In this way, the control can be adapted to changingcomponent values in the filter circuit, so that changing controlbehavior of the inverter can be compensated for.

The method steps of applying a voltage pulse, producing an oscillatingcircuit and determining and evaluating a current and/or voltage curvecan advantageously be repeated on a plurality of different pairs ofconductors. In this way, the effective filter inductance and filtercapacitance of each phase can be determined, particularly in the case ofmulti-phase inverters. If required, more detailed component values ofthe electrical components of the filter circuit can also be determinedin such an embodiment. Energy stored in an intermediate circuitcapacitor can be used for the voltage pulse.

At least one current value of an effective filter capacitance of thefilter circuit and at least one current value of an effective filterinductance of the filter circuit are advantageously determined as systemparameters. In the context of the present disclosure, “effective” filtercapacitance or filter inductance is either a currently determined valuecorresponding to an individual component of a filter circuit or acurrently determined value corresponding to a calculated component of anequivalent circuit of the filter circuit, depending on whether thefilter circuit in question is responsible for the determination of avalue of an actual component, or whether only the values of anequivalent circuit can be determined. “Effective” filter capacitances orfilter inductances can be further related to one another viamathematical dependencies in order to convert them into an effectivevalue that can be used for the control.

Depending on the topology of the inverter, different conductors can beused to determine the effective filter inductance and effective filtercapacitance. In an inverter with a conductor provided for the feedbackfrom the filter arrangement to the switching arrangement, one of theother conductors of the inverter is advantageously used as the firstconductor and the conductor provided for the feedback from the filterarrangement to the switching arrangement is used as the secondconductor. This allows the effective values of the individual phases tobe determined directly. Undesirable clamping can also be prevented inthis case when determining the effective filter inductance and filtercapacitance.

In the case of an inverter without a conductor provided for the feedbackfrom the filter arrangement to the switching arrangement, one of theavailable conductors of the inverter is advantageously used as the firstconductor and another of the available conductors of the inverter isused as the second conductor. In principle, this is also possible withan inverter with a neutral conductor connection. In order to preventundesired clamping, it can be provided that a conductor of the inverterthat is not used for determining the effective filter inductance andfilter capacitance is connected to an intermediate circuit potential viathe switching arrangement. A free-floating potential of this unusedconductor, which can lead to clamping, can thus be prevented. Thisenables the effective filter inductance and filter capacitance to bedetermined more precisely.

A conductor provided for the feedback from the filter arrangement to theswitching arrangement is a conductor which is either not connected tothe grid relay at all or is connected to a neutral conductor of thepower grid via the grid relay.

A resonant frequency of the oscillating circuit can advantageously bedetermined from the current and/or voltage curve, wherein a value of aneffective filter inductance can be determined according to the formula

$L_{m} = {\frac{1}{2 \cdot \pi \cdot f_{reso}} \cdot {\frac{U}{I}.}}$

A resonant frequency of the oscillating circuit can advantageously bedetermined from the current and/or voltage curve, wherein a value of aneffective filter capacitance is determined according to the formula

$C_{m} = {\frac{1}{2 \cdot \pi \cdot f_{reso}} \cdot {\frac{I}{U}.}}$

In an advantageous embodiment of the method, a decay behavior can bedetermined and taken into account in the control. In this way, forexample, the quality factor or the damping of the oscillating circuitcan be calculated. The associated ohmic resistance of the oscillatingcircuit can also be determined from this and can be used for thecontrol. The decay behavior can be taken into account, for example, whendesigning the controller of the control, for example when determiningthe controller parameters.

In a further advantageous embodiment, a closed oscillating circuit canbe produced immediately after the voltage pulse. As a result, anoscillation can be generated in the oscillating circuit even when thecapacitors are completely discharged. In connection with the presentdisclosure, “immediately after the voltage pulse” refers to a period oftime during which no relevant changes in the voltage and current stateshave occurred in the components involved. This is particularly the casewhen the corresponding values between the end of the voltage pulse andthe production of the oscillating circuit have changed by less than 10%,based on their total fluctuation range.

In a further aspect, the present disclosure relates to a correspondinglydesigned inverter for connection to a power grid via the grid relay.

The present disclosure also relates to a computer program with programcode for carrying out the method steps described above when the computerprogram is executed on a system control of an inverter. In this case,the computer program can advantageously determine a topology of theinverter before carrying out the method steps. The topology of theswitching arrangement of the inverter can be selected, for example, fromH5. HERIC, REFU, FB-DCBP, FB-ZVR, NPC, Conergy-NPC and topologiesrelated to these. The topologies denoted in this way are known in theart and therefore do not need to be explained in more detail here.

A person skilled in the art will be able to apply the teachingsdisclosed herein to all of the topologies mentioned. This also allowsthe same computer program to be used in a large number of differentinverters and also facilitates remote maintenance of the inverters.

The present invention is explained in more detail below with referenceto FIG. 1 to 11 , which schematically show advantageous embodiments ofthe invention by way of example and in a non-limiting manner. In thedrawings:

FIG. 1 shows an inverter in a schematic, generic representation,

FIG. 2 is a schematic view of a circuit of an inverter,

FIG. 3 shows part of a circuit of an inverter with an alternativeembodiment of a filter circuit,

FIG. 4 is a schematic representation of a further circuit of aninverter,

FIG. 5 is a schematic representation of a circuit of an inverter withfour conductors that can be connected to the line conductor and aneutral conductor of a power grid,

FIG. 6 is a schematic representation of a circuit of an inverter withthree conductors,

FIG. 7 is a further schematic representation of a circuit of an inverterwith four conductors,

FIG. 8 is a schematic representation of an equivalent circuit withequivalent total capacitances,

FIG. 9 shows a schematic representation of the voltage pulse, currentcurve and voltage curve as they can result, for example, for the methodin question,

FIG. 10 shows a control of an inverter and

FIG. 11 shows an equivalent circuit of a filter circuit.

FIG. 1 shows an inverter 1 which converts a direct current (DC) voltagegenerated by a DC voltage source 5 into an alternating current (AC)voltage which can be fed into a power grid 7 (also as an island grid).The inverter 1 of FIG. 1 has a three-phase design with three phases L1,L2, L3 and a neutral conductor N.

The DC voltage source 5 generates a potential difference U_(DC) which isapplied to a switching arrangement 2 of the inverter 1 via two inputsDC₁ and DC₂ on the DC side. Depending on the system, inputs DC₁, DC₂ onthe DC side can come directly from the DC voltage source 5 or from anupstream DC voltage converter or MPP tracker. The switching arrangement2 comprises, in a known manner, an intermediate circuit consisting of atleast one intermediate circuit capacitor C_(ZK) (not shown) and aplurality of semiconductor switches T, which are clocked via a systemcontrol 6 according to a modulation scheme. Freewheeling diodes D areusually arranged in parallel with the semiconductor switches T. Thesemiconductor switches T are often arranged in the form of half-bridgecircuits, wherein at least one half-bridge consisting of at least twoseries-connected semiconductor switches T is provided per phase. Thealternating voltage thus generated can be applied to correspondingconductors P via one or more conductor outputs W of the switchingarrangement 2. At least one conductor P is provided for each phase ofthe inverter, wherein a plurality of conductor outputs W can be combinedto form one phase (so-called interleaved inverter topologies).

The conductors P are routed via a filter circuit 3 to a grid relay 4,wherein with the grid relay 4 closed the conductors P of the inverter 1are connected to the corresponding conductors of the power grid 7 (i.e.,for example, phase or line conductors L₁, L₂, L₃ and neutral conductorN, and if necessary a protective conductor can also be taken intoaccount).

Conductors P, to which an alternating current can be applied via theswitching arrangement 2, are also referred to as “phase conductors” inconnection with the present description. In connection with the presentdisclosure, both phase conductors or line conductors and also neutralconductors are generally referred to as “conductors”. If a distinctionbetween phase conductors and neutral conductors is useful or necessary,this is explicitly stated in the text unless it is logically andcompellingly derivable from the context.

The filter circuit 3 generally comprises at least one filter inductanceLF (choke) which is arranged in a conductor P directly following thecorresponding conductor output W, and at least one filter capacitance CFwhich, preferably “behind” the filter inductance LF (i.e. between thefilter inductance LF and the grid relay 4), connects two conductors P toeach other. If necessary, in the case of multi-phase topologies, theconnection can be made via a star center point and a further filtercapacitance CF.

In connection with the present disclosure, components and elements thatappear multiple times in a similar or identical form in a drawing areidentified by a combination of capital letters identifying the element(e.g. DC-side input DC, semiconductor switch T, conductor output W,conductor P, filter inductance LF, filter capacitance CF, etc.) andnumbered by subscript indices. This differentiation is only for betterdistinguishability and is not to be interpreted restrictively.

Depending on the embodiment of the inverter 1, it can be equipped withtwo, three or four conductors P. Inverters with two conductors P₁, P₂can, for example, be connected to two phases L₁, L₂ of the power grid 7or to one phase L and the neutral conductor N. Inverters 1 with threeconductors P₁, P₂, P₃ can, for example, be connected to the three phasesL₁, L₂, L₃ of a three-phase power grid 7. Inverters 1 with fourconductors P₁, P₂, P₃, P₄ can, for example, be connected to the threephases L₁. L₂, L₃ of a three-phase power grid 7 and to its neutralconductor N.

The present disclosure is not limited to a specific topology of theinverter 1, in particular the switching arrangement 2 and the filtercircuit 3. Rather, the teachings disclosed herein can be applied to avariety of different topologies provided certain conditions are met,which are exemplified below with reference to some specific circuits setout in more detail.

The inverter 1, specifically the switching arrangement 2 of the inverter1, is controlled by a control 16, as shown in simplified form in FIG. 10. The control 16 is implemented in a system control 6, preferablymicroprocessor-based hardware, preferably as software. However, thesystem control 6 with the control 16 can also be designed as anintegrated circuit, for example as an application-specific integratedcircuit (ASIC) or field programmable gate array (FPGA), or as an analogcircuit.

A controller R(RP) with controller parameters RP is provided for thecontrol 16, wherein the controller parameters RP are adapted to thesystem parameters SP of the system to be controlled in order to achievethe desired control behavior. The filter circuit 3 influences thecontrol 16 of the inverter 1, so that the system parameters SP derivedfrom it are taken into account in the control 16, specifically in thecontroller R, for example in the form of a controller parameter RP or inthat the system parameters SP influence the value of a controllerparameter RP. In addition to the system parameters SP, which are derivedfrom the filter circuit 3, other system parameters can of course also betaken into account in the control 16. The design of a controller R witha predetermined control rule (e.g. PI controller, PID controller, etc.),on the basis of which the controller parameters RP and their dependenceon the system parameters SP are defined, is well known to a personskilled in the art so that it does not need to be discussed further. Inorder to control the inverter 1, specifically the switching arrangement2 or the semiconductor switches T of the switching arrangement 2, thecontroller R determines manipulated variables ST for the switchingarrangement 2 in dependence of a predefined setpoint variable SG, forexample a desired current per phase or a desired voltage per phase, forexample, switching commands for the semiconductor switch T or a dutycycle of a pulse width modulation (PWM) control, which are thenconverted into switching commands.

To control the inverter 1, for the filter circuit 3 for each phase ofthe inverter 1 use is made of an equivalent circuit consisting of aneffective filter inductance L_(m), which results from the at least onefilter inductance LF of the phase of the filter circuit 3 and thetopology of the filter circuit 3, and an effective filter capacitanceC_(m), which results from the at least one filter capacitance CF of thephase of the filter circuit 3 and the topology of the filter circuit 3(FIG. 11 ). The effective filter capacitance C_(m) and the effectivefilter inductance L_(m) are system parameters SP for the control 16 ofthe switching arrangement 2 and are determined from measured values ofan electrical current i(t) and an electrical voltage u(t) as describedbelow, for example in the system controller 6.

FIGS. 2 and 4 show an example of an inverter 1 for connection to a lineconductor L₁ and a neutral conductor N, for which system parameters suchas the effective filter capacitance C_(m) and the effective filterinductance L_(m) are determined.

FIG. 2 shows an embodiment of an inverter 1 with a conventional H-bridgecircuit, in which the potential difference U_(DC) between a highpotential (positive pole DC⁺) and a low potential (negative pole DC⁻)can be applied via four semiconductor switches T₁-T₄ to the twoconductor outputs W₁ and W₂ in different ways. In this embodiment, aphase L₁ is connected to the conductor output W₁ and the neutralconductor N of the power supply system 7 is connected to the conductoroutput W₂. During operation, the semiconductor switches T are controlledvia a system controller 6 according to a modulation scheme, wherein fourswitching states can be used:

-   -   positive current flow: T₁ and T₄ closed, T₃ and T₂ open    -   negative current flow: T₂ and T₃ closed, T₁ and T₄ open    -   zero volts over DC⁺: T₁ and T₃ closed, T₂ and T₄ open    -   zero volts over DC⁻: T₂ and T₄ closed, T₁ and T₃ open

Corresponding modulation schemes are well known in the art and it istherefore not necessary to describe them in detail here.

Irrespective of the topology, the circuit of the semiconductor switchesT generates a rectangular alternating current at the conductor outputs Waccording to a modulation scheme, and this alternating current must beconverted into a sine wave that runs as smoothly as possible before itis fed into the power grid 7. This is ensured by the filter circuit 3and the filter inductances LF₁, LF₂ and the filter capacitance CFprovided therein. The specific filter topology of the filter circuit 3and the specific values of the filter capacitances CF and filterinductances LF present in the filter give the filter circuit 3 aspecific filter behavior that can be described by the component values.The values depend on the particular frequency, wherein for the controlof the switching arrangement 2 not only the behavior at the frequency ofthe alternating current (typically, for example 16.7 Hz, 50 Hz, 60 Hz),but possibly also at interference frequencies and/or at frequencies thatare used for ripple control signals from the grid operator, can be takeninto account. When the inverter 1 is in operation, reactive currents,which are to be controlled by the control in the system control 6, flowthrough the filter capacitance CF. The filter capacitance CF and thefilter inductances LF1. LF2 thus influence the control of the inverter1.

In order to increase the accuracy of the reactive power value to beadjusted by the control and/or to optimize the control 16 of the systemcontrol 6, it is therefore essential to know the specific values of thesystem parameters SP for the control of the switching arrangement 2 asprecisely as possible. However, these system parameters SP, or thecomponents of the filter circuit 3, which are comprised in the systemparameters SP for the control, are subject to changes caused by aging orchanges in the environmental influences.

Depending on the topology of the filter circuit 3, the individualcomponent values of the filter capacitance(s) CF and the filterinductance(s) LF can only be determined with great effort. For thecontrol 16 according to the invention, an equivalent circuit 15 of thefilter circuit 3 with an effective filter capacitance C_(m) and aneffective filter inductance L_(m) is therefore used for each phase, asshown by way of example in FIG. 11 . In the equivalent circuit 15 forone phase of the inverter 1, all filter inductances LF of the phase arecombined in the effective filter inductance L_(m) or are disregarded(L_(emv)). Likewise, all filter capacitances CF of the phase arecombined in the effective filter capacitance C_(m). It is obvious thatthe way in which this combination has to be done depends on the topologyof the filter circuit 3. However, a person skilled in the art in thisfield can in any case determine the effective filter capacitance C_(m)and the effective filter inductance L_(m) of the equivalent circuit 15from a specific filter circuit 3. The control 16, specifically thecontroller parameters RP of the controller R of the control 16, isdesigned using the equivalent circuit 15.

A method by which the current system parameters SP of the filter circuit3 of the inverter 1 can be determined quickly, easily and precisely isdescribed below with reference to the circuit shown in FIG. 2 .Optimally, the method can be carried out immediately before the inverter1 is connected to the power grid 7 by closing the grid relay 4. Thesystem parameters SP can thus be determined, for example, regularly oras required before the grid relay 4 is closed or after the grid relay 4has been opened, whereby current system parameters SP can always bedetermined and taken into account in the control. A change in the systemparameters SP can therefore be continuously taken into account in thecontrol.

The method is carried out with the grid relay 4 open, i.e. the inverter1 is disconnected from the power grid 7, or from its line conductors L₁,L₂, L₃ and neutral conductor N, and all the semiconductor switches T areopen. By brief closure of the first and fourth semiconductor switches T₁and T₄, for example for a period of a few microseconds (e.g. 5microseconds), a voltage pulse is applied to the conductor outputs W₁and W₂ because an intermediate circuit voltage is applied for thisperiod. Alternatively, the voltage pulse can also be generated with theopposite polarity by closure of the second and third semiconductorswitches T₂ and T₃. Immediately afterwards, the conductor outputs W₁ andW₂ are connected in the switching arrangement 2, so that an oscillatingcircuit 8 is produced, which, starting from the first conductor outputW₁, runs via the first line P₁, the first filter inductance LF₁, thefilter capacitance CF, the second line P₂, the second filter inductorLF₂ to the second conductor output W₂ and is closed by the connectionbetween W₁ and W₂. The oscillating circuit 8 is indicated in FIG. 2 by adash-dot line. In the topology shown, the connection between W₁ and W₂can be established either by closing the “upper” semiconductor switchesT₁ and T₃ or by closing the “lower” semiconductor switches T₂ and T₄. Inmany topologies, however, it is also possible to connect the conductoroutputs W to one another without a potential difference being present atthem. If necessary, further semiconductor switches can be provided forthis purpose, such as the semiconductor switch T₅ shown in dashed lines,by which the bridge circuit can be separated from the positive pole DC*.

If the filter capacitance CF is charged before the voltage pulse isapplied, problems with overcurrent, for example if the capacitor ischarged too highly, can be prevented with an advantageous embodiment ofthe method. In principle, an overcurrent can be avoided with asufficiently short voltage pulse and, in addition, with an appropriatelyselected polarity. In a further embodiment of the method, the filtercapacitance CF is discharged before the voltage pulse is applied inorder to rule out an overcurrent and to be able to carry out a repeateddetermination of the system parameters under comparable conditions.

Even if the filter capacitance CF is completely discharged at thebeginning, a free oscillation forms immediately after the voltage pulsein the oscillating circuit 8, which can be determined as a current curvei(t) (e.g. current measurement 9 at the first or second filterinductance LF1, LF2) and a voltage curve u(t) (voltage measurement 13across the filter capacitance CF). The frequency of the current curve(and voltage curve) corresponds to the resonant frequency f_(reso) ofthe oscillating circuit 8. The resonant frequency f_(reso), the voltageamplitude U and the current amplitude I can thus be determined from thecurrent curve i(t) and the voltage curve u(t). An effective filterinductance L_(m) and an effective filter capacitance C_(m) can bedetermined for the filter circuit 3 from these values.

Using the law of conservation of energy applied to the oscillatingcircuit 8 with the equivalent circuit 15 of the filter circuit 3

L _(m) −I ² =C _(m) −U ²  (Eq. 1)

and the Hertz oscillation equation

$\begin{matrix}{f_{reso} = \frac{1}{2\pi\sqrt{L_{m} \cdot C_{m}}}} & \left( {{Eq}.2} \right)\end{matrix}$

results for the effective filter inductance

$\begin{matrix}{L_{m} = {\frac{1}{2 \cdot \pi \cdot f_{reso}} \cdot \frac{U}{I}}} & \left( {{Eq}.3} \right)\end{matrix}$

and the effective filter capacitance

$\begin{matrix}{C_{m} = {\frac{1}{2 \cdot \pi \cdot f_{reso}} \cdot {\frac{I}{U}.}}} & \left( {{Eq}.4} \right)\end{matrix}$

The effective filter inductance L_(m) and effective filter capacitanceC_(m) can result from a single or multiple physical component(s) of afilter circuit 3.

In the simple filter circuit 3 shown in FIG. 2 , the determinedeffective filter capacitance C_(m) corresponds to the current value ofthe filter capacitance CF. The effective filter inductance L_(m)corresponds to the sum of the two filter inductances LF₁ and LF₂.

The quality or the damping of the oscillating circuit 8 can becalculated by determining the decay behavior of the free oscillation.The associated ohmic resistance can also be determined from this.Quality or damping of the oscillating circuit 8 and ohmic resistance cansubsequently be used as further parameters for controlling the inverter1 for generating alternating current and alternating voltage or forcontrol optimization.

It is obvious that for the determination of the effective filtercapacitance C_(m) and the effective filter inductance L_(m) in aninverter 1 as in FIG. 2 , it is irrelevant whether it is connected to aphase conductor L₁ and a neutral conductor N (as in FIG. 2 ) or to twophase conductors L₁, L₂. When the grid relay 4 is open, there are onlythe conductors P of the inverter 1, which are used to form anoscillating circuit 8. Likewise, the specific design of the switchingarrangement 2 is not decisive for this.

As explained with reference to FIG. 3 , the above method can also beapplied to more complex filter circuits 3. The filter circuit 3comprises a filter inductance LF₁ in the first conductor P₁ and an EMCchoke L_(EMV1) as the second inductance for damping high-frequencyinterference. The second conductor P₂ also includes an EMC chokeL_(EMV2) in addition to the 25 second filter inductance LF₂. Instead ofa single filter capacitance CF, two parallel filter capacitances CF₁ andCF₂ are arranged in the filter circuit 3, wherein the two EMC chokesL_(EMV) are arranged between the first filter capacitance CF₁ and secondfilter capacitance CF₂. The remaining circuit of the inverter 1, whichis not shown in FIG. 3 , can correspond to FIG. 2 , for example.

The oscillating circuit 8 of FIG. 3 is thus divided between the twofilter inductances LF₁ and LF₂ into two parallel branches, wherein thefirst branch comprises the first filter capacitance CF₁ and the secondbranch comprises in series the first EMC choke L_(EMV1), the secondfilter capacitance CF₂ and the second EMC choke L_(EMV2).

With regard to the method described above for determining the systemparameters SP, the EMC chokes L_(EMC) can be disregarded in thecalculation. Since EMC chokes are usually designed for a significantlyhigher frequency than the filter inductances LF and are comparativelyvery small, this does not result in any disadvantages. The EMC chokeshave a negligible influence on the oscillating behavior of theoscillating circuit 8. The effective filter inductance L_(m) for theequivalent circuit 15 can thus be determined in a manner analogous tothe method described above, and again it corresponds to the sum of thetwo filter inductances LF₁ and LF₂. The effective filter capacitanceC_(m) can also be determined in an analogous manner and corresponds inthis case to the sum of the two parallel filter capacitances CF₁ andCF₂.

Knowledge of the effective filter capacitance C_(m) and effective filterinductance L_(m) is sufficient for the control 16 of the switchingarrangement 2, so that the additional effort for determining theindividual values of the parallel filter capacitances CF₁ and CF₂ andthe two filter inductances LF₁ and LF₂ is not necessary and can beomitted.

FIG. 4 shows a further example in the form of a multi-level inverter 1,wherein the switching arrangement 2 in this case corresponds to aso-called “NPC topology”. “NPC” stands for “neutral point clamped,”which means that a conductor output W of the switching arrangement 2, inthe illustrated case the second conductor output W₂, to which the secondconductor P₂ is connected, is connected to the intermediate circuitcenter point MP between two intermediate circuit capacitors C_(ZK1),C_(ZK2). The second conductor P₂ is connected to the filter circuit 3and, in this embodiment, is connected to the neutral conductor N of thepower grid 7 when the grid relay 4 is closed. Due to the topology of theswitching arrangement 2, more than two voltage levels (DC+, DC−) can nowbe set at the conductor output W₁ of the switching arrangement, to whichthe first conductor P₁ is connected. In the embodiment according to FIG.4 , the voltage level 0 can now also be set via the internalsemiconductor switches T₂, T₃ and the clamp diodes D₁₁, D₁₂. The foursemiconductor switches T₁ to T₄ of the NPC semiconductor bridge can beswitched into the following states in particular:

-   -   positive current flow: T₁ and T₂ closed, T₃ and T₄ open    -   negative current flow: T₃ and T₄ closed, T₁ and T₂ open    -   freewheeling (zero volt) via the two freewheeling diodes: T₂ and        T₃ closed, T₁ and T₄ open

To determine the system parameters SP, again a voltage pulse is appliedto a conductor output W₁, W₂ (for example by closing the two “upper”semiconductor switches T₁ and T₂ or the two lower semiconductor switchesT₃ and T₄) with the grid relay 4 open, because an intermediate circuitvoltage is present and immediately afterwards, by opening the two outersemiconductor switches T₁, T₄ and closing the two middle semiconductorswitches T₂ and T₃, the first conductor output W₁ is connected via oneof the two clamp diodes D₁₁, D₁₂ to the second conductor output W₂ andan oscillating circuit 8 is produced.

The system parameters SP are again determined according to the methoddescribed above, wherein only one filter inductance LF and one filtercapacitance CF have to be taken into account in this case. Thus, thevalues of the individual filter components can be determined directly.The effective filter capacitance C_(m) to be taken into account by thecontrol corresponds to the current value of the filter capacitance CFand the effective filter inductance L_(m) corresponds to the currentvalue of the filter inductance LF. However, more complex topologies ofthe filter circuit 3 are usually provided, so that such a simpleassignment is unusual and is only used for explanation.

With the aid of the teachings disclosed in connection with thedescription of FIG. 2 to 4 for single or two-phase inverters, thepresent disclosure can also be applied to numerous other one-, two- orthree-phase inverters which have a different topology. Examples of suchtopologies include, but are not limited to, those known as H5, HERIC,REFU, FB-DCBP, FB-ZVR, NPC, Conergy-NPC, and related or similartopologies.

The present teachings can advantageously be applied to three-phaseinverters 1 with a feedback from the filter circuit 3 into the switchingarrangement 2 by means of a conductor P₄, as is explained below by wayof example with reference to FIGS. 5 and 7 . The conductor P₄ used forthe feedback can also be connected to the neutral conductor N of thepower grid 7 when the grid relay 4 is closed. A significant advantagelies in the fact that the method for determining the system parametersSP can be used independently of the number of capacitances.

Three-phase inverters 1 can be made, for example, by combining threesingle-phase inverters. On the other hand, specific circuits forthree-phase inverters (with or without feedback) can also be used. Thestructure and the topology of one-, two- and three-phase inverters areknown per se to a person skilled in the art. The topologies that arelisted and described in connection with the inverters 1 mentioned abovecan also be used for three-phase systems by appropriate expansion of thecircuit. In principle, the present disclosure is not limited to specifictopologies unless specific technical reasons (such as an incompatibletopology) prevent implementation of the teachings disclosed herein.

FIG. 5 shows an inverter 1 with a switching arrangement 2 with fourconductor outputs We-W₄, wherein the corresponding first threeconductors P₁-P₃ are connected to the phase conductors L₁-L₃ of thepower grid 7 via the grid relay 4, and wherein the fourth conductor P₄is connected to the neutral conductor N (but this is not necessary). Inthe first three conductors P₁-P₃, a filter inductance LF₁-LF₃ is in eachcase provided directly following the corresponding conductor output Wand behind it a filter capacitance CF₁-CF₃ is in each case arrangedbetween the corresponding conductor P₁-P₃ (phase conductor) and thefourth conductor P₄ (which serves for feedback from the filter circuit 3to the switching arrangement 2 and is connected to the neutral conductorN when the grid relay 4 is closed), which are thus arranged in a stararrangement. The phase conductors P₁-P₃ can each be supplied with an (inparticular pulse width-modulated) AC voltage from the switchingarrangement 2. The fourth conductor P₄ for feedback can be connected inthe switching arrangement 2 to an intermediate circuit center point MPbetween two intermediate circuit capacitances C_(ZK1), C_(ZK2)(comparable to the connection P₂ in FIG. 4 ).

The method for determining system parameters disclosed above inconnection with single-phase or two-phase inverters 1 is basicallysuitable for inverters 1 that can produce an oscillating circuit 8 via afilter circuit 3 between two outputs of the switching bridge and can,for example, be applied to the filter topology of FIG. 5 .

To determine the system parameters SP, one of the first three conductoroutputs W₁-W₃ is first subjected to a voltage pulse. Immediately afterthe voltage pulse, starting from this conductor output, an oscillatingcircuit 8′ is built up via the associated conductor P, via the filtercircuit 3 and the conductor P₄ provided for the feedback and connectedto the conductor output W₄. The oscillating circuit 8′ can be routed viathe corresponding filter inductance LF, the corresponding filtercapacitance CF and the fourth conductor P₄ provided for the feedback, inthat the corresponding conductor output W₁-W₃ is connected to the fourthconductor output W₄ via the switching arrangement 2. Such an oscillatingcircuit 8′ is shown in FIG. 5 by the dash-dot line between the conductoroutput W₁ and the conductor output W₄. The method is analogously alsoused for the other conductor outputs W₁-W₃. In this way, the effectivefilter capacitance C_(m) and effective filter inductance L of each phaseof the inverter 1 are obtained as described above.

FIG. 7 shows a further embodiment of a three-phase inverter 1 with aconductor P₄ provided for the feedback from the filter circuit 3 to theswitching arrangement 2, the switching arrangement 2 corresponding to a3L-NPC (three level neutral point clamped) topology. Of the fourconductors P₁-P₄, when the grid relay 4 is closed, the first threeconductors P₁-P₃ are each assigned to a phase L₁-L₃ of the power grid 7(phase conductor), and the fourth conductor P₄ is assigned to theneutral conductor N and connected to an intermediate circuit centerpoint MP of the intermediate circuit. However, this connection of theconductor P₄ to a neutral conductor N when the grid relay 4 is closed isnot mandatory.

The filter circuit 3 comprises a filter inductance LF₁-LF₃ for eachconductor P₁, P₂, P₃. Furthermore, an EMC choke L_(E)v is provided ineach conductor P₁, P₂, P₃, wherein the EMC chokes L_(EMV) can again bedisregarded for determination of the system parameters SP, as alreadyexplained. A first star connection with three filter capacitancesCF₁-CF₃ is arranged between the filter inductances LF₁-LF₃ and the EMCchokes L_(EMV), and a second star connection with three further filtercapacitances CF₄-CF₆ is arranged after the EMC chokes L_(EMV). The starcenter points of the two star connections are each connected to thefourth conductor P₄ provided for the feedback to the switchingarrangement 2.

With the switching arrangement 2 shown, either a positive potential(positive pole DC⁺), a negative potential (negative pole DC⁻) or theintermediate neutral potential of the intermediate circuit center pointMP of the intermediate circuit can be applied in the form of a voltagepulse to each of the three conductors L (i.e. the three first conductorsP₁-P₃), i.e. for each phase of the inverter 1 (the correspondingswitching of the semiconductor switches T corresponds to the proceduredescribed in connection with FIG. 4 ). Thus, for each phase of theinverter 1, an oscillating circuit 8 can be produced which, startingfrom the center point MP, runs via the two clamp diodes D_(x1), D_(x2)and the two middle semiconductor switches T_(x2), T_(x3) of the relevantphase, the conductor output W of the relevant phase, the filterinductance LF and the two parallel filter capacitances CF of therelevant phase and the fourth conductor P4 for feedback, back to theintermediate circuit center point MP.

Either a positive voltage pulse (semiconductor switches T_(x1) andT_(x2) closed) or a negative voltage pulse (semiconductor switchesT_(x3) and T_(x4) closed) can be applied as the voltage pulse. Afterthat, the oscillating circuit 8 is again produced as described above andthe current and/or voltage curve is measured and evaluated. This processis performed for each of the three phases. As a result, the values ofthe effective filter inductance L_(m) and the effective filtercapacitance C_(m) for each phase can be determined as system parametersSP.

In FIG. 7 , the oscillating circuit 8 for the first phase, i.e. via theconductor P₁, is represented by a dash-dot line. The evaluation of thebehavior of this oscillating circuit 8 allows to determine the value ofthe effective filter inductance L_(m) in the form of the first filterinductance LF1 and of the effective filter capacitance C_(m), which inthis case corresponds to the sum of the two parallel filter capacitancesCF₁ and CF₄ of this oscillating circuit 8 and is determined as a totalcapacitance. The effective filter inductances L_(m) and effective filtercapacitances C_(m) of the other two phases are determined analogously.Due to the connection of the capacitor star point of the filtercapacitors CF to the intermediate circuit center point MP, the effectivefilter inductances L_(m) and effective filter capacitances C_(m) of theindividual phases can be easily determined with an oscillating circuit 8formed between one of the conductor outputs W₁, W₂, W₃ of the conductorsP₁, P₂, P₃ and the conductor output W₄ of the conductor P₄ provided forthe feedback.

On the other hand, two “phase outputs” (i.e. two of the first threeconductor outputs W₁-W₃) can also be connected to each other via theswitching arrangement 2. This is possible for the examples in FIG. 5 andFIG. 7 , but it is particularly suitable for the example in FIG. 6 andFIG. 8 , since here there is no feedback from the filter circuit 3 tothe switching arrangement 2. The corresponding oscillating circuit 8″ issomewhat more complex and is shown by way of example in FIG. 5 and FIG.6 by a dash-dot-dot line between the conductor outputs W₂ and W₃. InFIG. 5 , this exemplary oscillating circuit 8″ comprises the secondfilter inductance LF₂, the second filter capacitance CF₂, the thirdfilter capacitance CF₃ and the third filter inductance LF₃ in a seriesconnection.

Thus, in an embodiment according to FIG. 5 or FIG. 7 , six differentoscillating circuits 8′, 8″ are available (while, for example, in thecase of the circuit shown in FIG. 6 described below, three differentoscillating circuits 8″ are available). In the simple case shown in FIG.5 (oscillating circuit 8′), the exact parameters of all filtercapacitances CF and filter inductances LF can also be determined exactlyusing an evaluation of the three oscillating circuits 8′ formed via theconductor L₄ provided for the feedback (i.e. the fourth conductor outputW₄). For the circuit in FIG. 6 , for example, no parameters related toindividual filter capacitances can be determined by the methodsdisclosed here, but the effective filter inductance L_(m) and effectivefilter capacitance C_(m) can be determined, which is sufficient for thecontrol 16 of the inverter 1. The variant of the method that ispreferred in each case thus depends on the specific topology of theswitching arrangement 2 or the filter circuit 3 connected thereto.

If further filter capacitances CF are present in the filter circuit 3(as is Indicated, for example, in FIG. 5 by the filter capacitances CF₄to CF₆ drawn in dashed lines), the sum value of parallel filtercapacitances in the oscillating circuit can again be determined as theeffective filter capacitance C_(m).

FIG. 6 shows a three-phase inverter 1 which, however, has a filtercircuit 3 that does not have a feedback to the switching arrangement 2.Three conductor outputs W₁-W₃ are each connected to a conductor P₁-P₃,wherein the conductors can each be connected to a phase L₁-L₃ of thepower grid 7.

The filter circuit 3 comprises (in the direction from the conductoroutputs W to the grid relays 4) three filter inductances LF-LF₃ (one perconductor), a star connection with three filter capacitances CF₁-CF₃ anda free star point, three EMC chokes L_(EMV1)-L_(EMV3) (one perconductor) and three filter capacitances CF₄-CF₆ in delta connection.When the grid relay 4 is closed, the free star point could also beconnected to a neutral conductor N of the power grid 7.

Any combination of star and/or delta connections of capacitors can berepresented as an equivalent circuit in the form of a pure starconnection or in the form of a pure delta connection. In this sense, forthe star-delta connection in FIG. 6 , the delta connection made up ofCF₄, CF₅, CF₆ can be replaced by an equivalent star connection, forexample. This results in two star connections with the star connectionof CF₁, CF₂, CF₃, which in turn can be combined into a single equivalentstar connection. This results in an equivalent circuit 15 for FIG. 6with the effective filter capacitances C_(m1), C_(m2), C_(m3) asequivalent total capacitances according to FIG. 8 .

For the determination of the system parameters SP in FIG. 6 , one of thethree conductor outputs W₁-W₃ is again first subjected to a voltagepulse. Immediately after the voltage pulse, starting from this conductoroutput, an oscillating circuit 8″ is built up via the filter circuit 3,wherein this conductor output is short-circuited via the switchingarrangement 2 with one of the two remaining conductor outputs W₁-W₃. Asbefore, the corresponding oscillating circuit 8″ runs as the voltagepulse before in parallel over the star connection and the deltaconnection of the filter circuit 3, wherein the EMC chokes L_(EMV) areagain disregarded. The effective filter capacitances C_(m1), C_(m2),C_(m3) of the equivalent circuit 15 in FIG. 6 are obtained bycalculation with the determination of the equivalent total capacitancesin FIG. 8 .

If for FIG. 6 a star connection is used as an equivalent circuit 15, asin FIG. 8 , three total capacitances C_(m12), C_(m23), C_(m31) can bedetermined using the method described above. In the case of a starequivalent circuit, these correspond to the total capacitances of theseries connection of two equivalent total capacitances (C_(m1), C_(m2),C_(m3)). The following system of equations can be set up with thedetermined total capacitances C_(m12), C_(m23), C_(m31):

$C_{m12} = \frac{C_{m1} \star C_{m2}}{C_{m1} + C_{m2}}$$C_{m23} = \frac{C_{m2} \star C_{m3}}{C_{m2} + C_{m4}}$$C_{m31} = \frac{C_{m1} \star C_{m3}}{C_{m1} + C_{m3}}$

The effective filter capacitances C_(m1), C_(m2), C_(m3) of theindividual phases can be determined from this system of equations bysolving the above equation system for the effective filter capacitancesC_(m1), C_(m2), C_(m3), which leads to the following equations:

$C_{m1} = \frac{2 \star C_{m12} \star C_{m23} \star C_{m31}}{{C_{m12} \star C_{m23}} - {C_{m12} \star C_{m31}} + {C_{m23} \star C_{m31}}}$$C_{m2} = \frac{2 \star C_{m12} \star C_{m23} \star C_{m31}}{{C_{m12} \star C_{m23}} - {C_{m12} \star C_{m31}} - {C_{m23} \star C_{m31}}}$$C_{m3} = \frac{2 \star C_{m12} \star C_{m23} \star C_{m31}}{{C_{m12} \star C_{m23}} + {C_{m12} \star C_{m31}} - {C_{m23} \star C_{m31}}}$

For illustration, FIG. 9 shows a voltage pulse 10 in a first diagram,and the response of the oscillating circuit 8″ to the voltage pulse 10,for example according to FIG. 8 , is shown in two further diagrams. Inthe example shown, the oscillating circuit 8″ consists of the effectivefilter capacitances C_(m2). C_(m3) and the filter inductances LF₂, LF₃(which in this example correspond to the effective filter inductancesL_(m)). Among other things, a resonant frequency (f_(reso)=1593.8 Hz), acurrent amplitude (I=32.9 A) and a voltage amplitude (U=32.9 V) can bedetermined via the determined current curve 11 or the voltage curve 12.Inserted into Eq. 4, this results in a value of C_(m23)=100 μF for theeffective total capacitance C_(m23). With this procedure C_(m12) andC_(m31) can be determined in the same way. The effective filtercapacitances C_(m1), C_(m2), C_(m3) can thus be determined using theabove system of equations.

The effective filter capacitances C_(m1), C_(m2), C_(m3) correspond tothe current values required for the control and can in turn betransformed into effective filter capacitances for a delta equivalentcircuit of capacitors by means of star-delta transformation if required,if a control 16 requires the effective filter capacitances C_(m1),C_(m2), C_(m3) in this form.

According to FIG. 8 , three total inductances L_(m12), L_(m23), L_(m31)can also be determined using the method described above. With these, thefollowing system of equations can be set up:

L _(m12) =LF ₁ +LF ₂

L _(m23) =LF ₃ +LF ₂

L _(m23) =LF ₁ +LF ₃

Analogously to the determination of the total capacitance C_(m23), thetotal inductance L_(m23) can be calculated using Eq. 3 and the valuesdetermined according to FIG. 9 with L_(m23)=100 μH. The totalinductances L_(m12) and L_(m31) can be determined in the same way withthe method. This means that the system of equations above can be solvedfor the effective filter inductances L_(m1)=LF₁, L_(m2)=LF₂ andL_(m3)=LF₃ and the current values can be calculated:

$L_{m1} = {\frac{1}{2}\left( {L_{m12} - L_{m23} + L_{m31}} \right)}$$L_{m2} = {\frac{1}{2}\left( {L_{m12} + L_{m23} - L_{m31}} \right)}$$L_{m3} = {\frac{1}{2}\left( {{- L_{m12}} + L_{m23} + L_{m31}} \right)}$

The exemplary voltage pulse 10 in FIG. 9 is generated by the switchingarrangement 2 of an inverter 1 at the time t=1 ms with u1=600V. In thepreceding idle state (e.g. before normal operation), the filtercapacitor(s) affected by the voltage pulse 10 are discharged in thisexample (u2=0 V) and no current flows through the affected filterinductance(s) (i=0 A). The current curve 11 shows the current flowthrough the effective filter inductance L_(m). At maximum current, thevoltage measured across the effective filter capacitance C_(m) isminimal or zero according to the voltage curve 12 and the entire energyof the oscillating circuit 8 is stored in the magnetic field of theeffective filter inductance L_(m). The oscillating circuit 8 is formedimmediately after the pulse. Shortly before t=1.2 ms, the current flow11 through the effective filter inductance L_(m) is minimal or zero andthe entire energy of the oscillating circuit 8 is stored in theeffective filter capacitance C_(m), wherein the voltage curve 12 has themaximum voltage. Eq. 1 is based on this relationship.

The current values for the effective filter capacitance C_(m) and thecurrent values for the effective filter inductance L_(m) can thereforebe determined based on the systems of equations.

In the case of inverters 1 with three or more phase conductors P₁, P₂,P₃ and without a defined zero state, such as a topology as in FIG. 6 ,however, there can be certain difficulties in the determination of theeffective filter capacitances C_(m) and the effective filter inductancesL_(m) of the individual phases. In the test case shown in FIG. 6 , theoscillating voltage is divided at the filter capacitances CF₂, CF₃. Inthe ideal case, both filter capacitances CF₂, CF₃ are the same size, asa result of which the potential which was applied by the switchingarrangement 2 for the voltage pulse (e.g. DC+ or DC−) is established atthe common capacitor star point.

However, if the two filter capacitances CF₂, CF₃ are not the same size,which is quite possible in reality, this no longer applies. In thiscase, the potential of the capacitor star point oscillates. Thisoscillation of the potential of the capacitor star point also causes thepotential at the conductor output W₁ to oscillate via the filterinductance LF₁ because W₁ is to be regarded as open for this test case.This undesired oscillation of the conductor output W₁ can lead to thepotential of the positive intermediate circuit voltage DC+ beingexceeded or the potential of the negative intermediate circuit voltageDC− being undershot. In both cases, one of the freewheeling diodes D ofthe semiconductor switches T in the switching branch of the conductoroutput W₁ would become conductive, as a result of which a current wouldflow into the intermediate circuit and would falsify the measurement ofcurrent and voltage, resulting in an inaccurate determination of theeffective filter capacitances C_(m) and the effective filter inductancesL_(m). This effect is called “clamping”. This clamping effect isindependent of whether the filter capacitors CF₁, CF₂, CF₃ are arrangedin a star or delta connection and can also occur with filter inductancesLF₁, LF₂, LF₃ of different sizes. It is obvious that this clamping canalso occur in the case of oscillating circuits 8″ that are formed otherthan those shown in the test case in FIG. 6 . By measuring the currentthrough the filter inductances LF (which will be implemented in any casefor the method according to the invention), such clamping can bedetected and, if necessary, taken into account for the control.

In order to prevent such clamping, a topology with a conductor P₄ usedas feedback from the filter circuit 3 to the switching arrangement 2,which is connected to a defined zero state, can be used. Such a topologywould be, for example, a topology as shown in FIG. 7 , where theconductor P₄ is connected to the intermediate circuit center point MP,or as in FIG. 5 , if the conductor P₄ is also connected to anintermediate circuit center point MP. This can prevent an oscillatingpotential at the capacitor star point.

Another possibility for preventing clamping would be not to leave thepotential of the phase unused for the respective test case (the phase P₁in the test case of FIG. 6 ) free-floating, but to connect via theswitching arrangement to the potential which was used for the voltagepulse (i.e. DC+ or DC−), which prevents the potential from oscillatingat the associated conductor output W. However, due to resulting circuit,the filter inductance LF₁ and filter capacitances CF₁ of the phase P,unused for the test case are involved in the oscillating circuit 8″ ofthe test case and must therefore be taken into account when determiningthe effective filter capacitances C_(m) and effective filter inductancesL_(m). This would make the equations explained above somewhat morecomplex, but would not change the basic procedure for determining theeffective filter capacitances C_(m) and effective filter inductancesL_(m).

The decay behavior of the oscillating circuit is not shown in FIG. 9 ,but the parameters for it can also be determined on the basis of thisand can also be taken into account in the control. Due to the decaybehavior, it is important for the accuracy of the determined systemparameters SP to determine the voltage and current amplitude U, I of thevoltage and current curve 11, 12 as closely as possible in time, so thatan energy balance according to Eq. 1 can be assumed. The period of timedepends on the resonant frequency and the decay behavior and is, forexample, one or a few period duration(s) of the resonant frequency. Themethod illustrated in FIG. 9 can also be applied to all the othertopologies previously disclosed and also to other topologies.

For the purpose of the control 16 of the energy conversion or theswitching arrangement 2 of the inverter 1, system parameters SP in theform of effective filter capacitances C_(m) and/or effective filterinductances L_(m) of a filter circuit 3 are sufficient. It is notnecessary to determine individual component values, but this can resultin certain cases. A permitted value range can also be defined for valuesof effective filter capacitances C_(m), wherein error messages or errorstates of an inverter 1 can be defined for values outside the permittedvalue range.

1. A method for controlling a switching arrangement of an inverter witha control, wherein the inverter has the switching arrangement, a filtercircuit and a grid relay and the control takes system parameters of thefilter circuit of the inverter into account, wherein the switchingarrangement has at least two conductor outputs and each conductor outputis connected to the filter circuit by a conductor and the conductors ofthe inverter provided for connection to a power grid are connected tothe grid relay, wherein the filter circuit is formed from at least onefilter inductance arranged in a conductor and at least one filtercapacitance which connects two conductors to one another, and for thefilter circuit use is made of an equivalent circuit consisting of aneffective filter inductance (L_(m)), which results from the at least onefilter inductance of the filter circuit and the topology of the filtercircuit, and an effective filter capacitance (C_(m)), which results fromthe at least one filter capacitance of the filter circuit and thetopology of the filter circuit, wherein the effective filter inductance(L_(m)) and the effective filter capacitance (C_(m)) are used as systemparameters, and the method comprises the following, which are carriedout with the grid relay open: applying a voltage pulse between a firstconductor output and a second conductor output, connecting this firstconductor output and this second conductor output via the switchingarrangement to produce a closed oscillating circuit which runs, startingfrom the first conductor output and the first conductor connected to thefirst conductor output via the filter circuit and the second conductorto the second conductor output connected to the second conductor,determining a current curve and/or voltage curve in the oscillatingcircuit, evaluating the current curve and/or voltage curve to determineat least one current value of the effective filter inductance (L_(m))and the effective filter capacitance (C_(m)) of the filter circuit assystem parameters of the filter circuit, and the switching arrangementof the inverter is controlled with closed grid relay taking into accountthe determined current values of the effective filter inductance (L_(m))and the effective filter capacitance (C_(m)) of the filter circuit. 2.The method according to claim 1, wherein the method steps of applying avoltage pulse, producing an oscillating circuit and determining andevaluating a current and/or voltage curve are repeated on a plurality ofdifferent pairs of conductors.
 3. The method according to claim 1,wherein in an inverter with a conductor provided for a feedback from thefilter arrangement to the switching arrangement, one of the otherconductors of the inverter is used as the first conductor, and theconductor provided for the feedback from the filter arrangement to theswitching arrangement is used as the second conductor.
 4. The methodaccording to claim 1, wherein in the case of an inverter without aconductor provided for a feedback from the filter arrangement to theswitching arrangement, one of the available conductors of the inverteris used as the first conductor, and another of the available conductorsof the inverter is used as the second conductor.
 5. The method accordingto claim 4, wherein a conductor of the inverter that is not used fordetermining the effective filter inductance and filter capacitance isconnected to an intermediate circuit potential via the switchingarrangement.
 6. The method according to claim 1, wherein a resonantfrequency (f_(reso)) of the oscillating circuit is determined from thecurrent curve and voltage curve wherein according to the formula$L_{m} = {\frac{1}{2 \cdot \pi \cdot f_{reso}} \cdot \frac{U}{I}}$ avalue of the effective filter inductance (L_(m)) is determined.
 7. Themethod according to claim 1, wherein a resonant frequency (f_(reso)) ofthe oscillating circuit is determined from the current and voltagecurve, wherein according to the formula$C_{m} = {\frac{1}{2 \cdot \pi \cdot f_{reso}} \cdot \frac{I}{U}}$ avalue of the effective filter capacitance (C_(m)) is determined.
 8. Themethod according to claim 1, wherein a decay behavior of the currentcurve and/or voltage curve in the oscillating circuit is determined andthe decay behavior is taken into account in the control.
 9. The methodaccording to claim 1, wherein a closed oscillating circuit is producedimmediately after the voltage pulse.
 10. An inverter comprising with aswitching arrangement with semiconductor switches and a system control,in which a control with a controller with controller parameters forcontrolling the switching of the semiconductor switches is implemented,wherein the inverter further comprises a filter circuit and a grid relayand the control is configured to take system parameters of the filtercircuit of the inverter into account, wherein at least two conductoroutputs are provided on the switching arrangement and each conductoroutput is connected to the filter circuit via a conductor, and theconductors of the inverter provided for connection to a power grid areconnected to the grid relay, wherein at least one filter inductancearranged in a conductor and at least one filter capacitance whichconnects two conductors to one another are provided in the filtercircuit, wherein an effective filter inductance, which results from theat least one filter inductance of the filter circuit and the topology ofthe filter circuit, and an effective filter capacitance, which resultsfrom the at least one filter capacitance of the filter circuit and thetopology of the filter circuit, of an equivalent circuit of the filtercircuit are provided as system parameter, wherein the switchingarrangement is configured to apply a voltage pulse between a firstconductor output and a second conductor output when the grid relay isopen, wherein the switching arrangement is configured to connect thisfirst conductor output and this second conductor output after theapplication of the voltage pulse to produce a closed oscillating circuitwhich runs, starting from the first conductor output and the firstconductor connected thereto via the filter circuit and the secondconductor to the second conductor output connected thereto, the systemcontrol is configured to determine a current value of the effectivefilter inductance and the effective filter capacitance of the filtercircuit from an electrical current determined in the oscillating circuitand/or from an electrical voltage determined in the oscillating circuit,and wherein the system control is configured to control the inverterwith closed grid relay with the determined current values of theeffective filter inductance and the effective filter capacitance of thefilter circuit.
 11. A computer program with program code for carryingout method according to claim 1, when the computer program is executedon a system control of an inverter.